Radio telecommunications systems using code division multiple access (CDMA) transmit multiple channels simultaneously in the same frequency band.
As shown in FIG. 1, a baseband signal 2 for transmission, which as used herein includes digital signals which may have been processed by a processor 4 to compress them, is modulated 6 using a modulation scheme such as Quaternary phase shift keying (QPSK) or quadrature amplitude modulation (QAM) so as to define a sequence of “symbols” that are to be transmitted.
The symbols which have both an in phase and imaginary component occur at a rate known as a symbol rate. The symbols from the modulator undergo two further processing operations prior to transmission.
The symbols are operated on by a spreading code 8 so as to spread the data from each symbol. The spread data is then further multiplied by a scramble code 10 which is specific to the cell that the mobile or base station is operating in. The result of these processes is to generate “chips” which are then transmitted following up-conversion 12 to the desired transmit frequency.
The spreading codes are selected such that they make the symbols mutually orthogonal. This condition applies whilst all of the chips are in time alignment (which is easy to achieve at the transmitter) but the occurrence of multiple transmission paths in the propagation channel between the transmitter and receiver can result in multiple versions of the same transmit sequence of chips arriving with different time delays and amplitudes at the receiver, as schematically shown in FIG. 2. To put the multi-path propagation problem in context, the chip rate for UMTS is 3.84 M chips per second. This means that a radio wave carrying the data propagates around 75 metres in one chip period. Consequently any path differences in excess of 75 metres allow two completely different chips to arrive at the receiver at the same time. The receiver needs to be able to undo the effects of the multi-path distortion to recover the transmitted data. The different propagation paths, as illustrated by the vertical lines 15 are sometimes referred to as “rays”.
One technique for recovering the transmitted data is to use a rake receiver to seek to re-align the various time displaced versions of the original signal.
A rake receiver is schematically shown in FIG. 3. It comprises a plurality of individual processing channels 30-1, 30-2 to 30-N, known as fingers. Each finger allows the relative time alignment between the received signal and a de-spreading code to be adjusted. This enables signal power from each significant transmission path to be recovered and brought into time alignment.
In prior art rake receivers, each finger comprises a plurality of correlators so as to integrate the correlation product of the incoming signal with the de-spreading and descrambling code.
FIG. 4 schematically illustrates the functionality within known fingers of rake receiver. Each finger comprises several correlators. The correlators act to correlate the down converted and digitised signal RxI and RxQ provided by a radio frequency front end with descrambling signals provided by a local descrambling code generator which is known to the person skilled in the art and need not be described in detail here. The scrambling/descrambling codes are selected in a known manner and have the property that their autocorrelation function is large if the codes are in correct temporal alignment and substantially zero otherwise.
Each finger has a respective delay set up by a process for estimating the channel response and assigned to the finger such that it is responsible for recovering a signal from a specific one of the multiple transmission paths. Once set up, the finger uses a closed loop control to make sure that it is properly time aligned to within ⅛ chip with the signal it is seeking to receive.
Three correlators are used in the closed loop timing control and are provided with versions of the input signal, each slightly offset in time. An “early” correlator 40 receives an input directly from the RF front end. A delay of ½ chip is provided by a first delay element 42 and the output of the delay element is provided to an “on time” correlator 44 and data extraction correlators Data 0, data 1 to data N. The output of the delay element is further provided to a second ½ chip delay element 46 who's output is provided to a “late” correlator 46.
Thus, when compared to the “on time” correlator the “early” correlator sees a time advanced version of the input end the late correlator sees a time delayed version of the input.
The “early”, “on time” and “late” correlators examine the data to identify a known sequence called the common pilot channel (CPICH) which is also used in the process of characterising the communications channel. By correlating with the CPICH and filtering it the receiver can use the relative powers of the early, on time and late correlators to check that it is properly aligned with the time delayed version of the signal that the particular finger has been assigned to, and to adjust it's timing if necessary by modifying the timing of the de-spreading sequence with respect to an internal reference time, or modifying the timing of the received signal with respect to the despread (sometimes referred to as derotated) sequence. The process of adjusting the time to maintain alignment with the “on time” symbols is commonly referred to as a “delay locked loop”. Only the magnitude (or power) of the signals at the outputs of the correlators is of use in time aligning the finger using the delay locked loop. The “on time” correlated output can then also be used as the phase reference to de-rotate the data at the correlator outputs (as part of a maximum ratio combining process where both phase and magnitude are considered), or looked at in a different way, to remove phase uncertainty from the data. If the “on time” phase reference is not used, then another time multiplexed pilot phase reference can be used instead in order to achieve the same objective.
The data could be decoded by a single channel but in a terminal conforming to or having capabilities up to the high speed data packet access (HSPDA) category 6 standard the data could be in any one of twelve data channels. To ensure data recovery the finger includes a correlator for each of these twelve channels.
For transmit diversity a time multiplexed pilot is used and as these are only ever carried on one physical data channel the correlator for this channel can be succeeded by two further correlators arranged to detect the A and B pilots, respectively.
This architecture repeated across several fingers of the rake receiver can take up significant space on a silicon die.
As briefly alluded to herein before, the channel response must be estimated prior to setting up the appropriate delay for each finger of the rake receiver. The channel response is estimated by a path searcher which, in essence, looks across all possible delay times within a search window to correlate a received signal having the CPICH therein with various time delayed versions of the despread and descramble codes. Therefore there is similarity between the functionality provided as part of the rake receiver to check that it remains time aligned using the power bearing components of the “early”, “on time” and “late” CPICH which has undergone multi-path distortion and the path searcher which is used to characterise the transmission path and hence to identify those power bearing components with the different time delays which should be realigned at the receiver in order to improve the signal to noise ratio.
The prior art rake receiver architecture typically includes 9 fingers and a shared combiner. Each finger is instantiated separately despite sharing the same structure. The prior art receiver also has a separate path searcher, and possibly a plurality of them.
The prior art finger (i.e. not complying with HSPDA category 6) consists of anywhere between five and seven correlators (to integrate and dump). Five of the correlators have a filter engine attached to provide a moving average of the symbols it correlates against. The pilot (CPICH and TMP) symbols are used for phase rotation. Filtering provides a less noisy version of these pilot signals. The filtering is varied to account for the physical speed of the mobile device, and can be done over two pilot symbols for a fast moving device and 16 pilot symbols for a slow moving device.